It is well-known in the ocular glass field to use inorganic colloids in scratch-resistant and/or abrasion-resistant coatings (also referred to as “hard coatings”) for improving the surface properties of organic glasses and/or for modifying the refractive index thereof. Such colloids are also used in anti-reflection coatings as well as in impact-resistant primers.
Zirconia, or zirconium dioxide, is one of these known colloids, because it provides a number of useful mechanical and physical properties, including high fracture toughness, mechanical strength and hardness, low thermal conductivity, high refractive index and transparency in the visible and infra-red spectral regions. However, it may be difficult to produce in a simple and reproducible manner a colloidal suspension of zirconia with high crystallinity in the form of a stable homogeneous dispersion of nanoparticles (having an average particle size of less than 20 nm) at high solids content (about 20 wt. %), and also to avoid their aggregation both during the preparation of the sot and during incorporation into a polymer matrix, for instance an epoxysilane matrix. Such a homogeneous dispersion is however required for the formation of a transparent zirconia-containing composite film and it directly affects the haze and transparency of the optical coating. This stable homogeneous dispersion may be expressed by the zeta potential of the suspension, which absolute value should be of at least 30 mV, and by its viscosity, which should be less than 10 cPs at 25° C. (no gelling should occur). The transparency is ensured by the low particle size with uniform and narrow size distribution. The high refractive index is related to the high crystallinity of the colloidal zirconia.
Several methods have already been proposed to prepare colloidal zirconia.
One known method for preparing acid zirconia colloids was to directly heat an aqueous solution of acid zirconium sources, such as zirconium oxychloride or zirconium nitrate, as disclosed in EP 0 229 657 and U.S. Pat. No. 2,984,928. Due to the strong acidic system, this kind of method results in zirconia colloids having a low crystallinity and in an incomplete reaction of the precursor, even through long heat treatment time of several days. This low crystallinity of the product would affect the refractive index of zirconia colloids, and directly affects their application in optical coatings.
Still another approach described in CN-101613123 and by HUANG Y. et al. in Wujiyan Gongye, 37(7), 15-17 (2005) has consisted in reacting a zirconium source, such as zirconium oxychloride, with an alkaline precipitant such as ammonia to obtain a zirconium hydroxide precipitate which is then subjected to hydrothermal treatment in the presence of a mineralizer, such as NH4Cl, a hydroxide or a carbonate, for instance at a temperature of 125-350° C. After drying, a zirconia powder having a small and/or uniform particle size and a good dispersibility is said to be obtained. However, these methods lead to dry powders. When redispersed in a solvent, the particles will remain aggregated in some extent, which will be detrimental to the transparency of the colloidal suspensions obtained.
Similarly, US 2010/0144918 discloses a method for preparing colloidal zirconia suspensions, starting from a zirconium hydroxide suspension which may be prepared by adding a basic precipitant such as ammonia to zirconium oxychloride. Zirconium hydroxide is then reacted with an inorganic acid such as HCl before hydrothermally treating the suspension at 150-200° C. This method results in a zirconia sol having a dry matter content of 3-10% only, in which. zirconia is provided as mainly tetragonal crystals.
It has now been found that the addition of a specific type of mineralizer to zirconium oxychloride, before subjecting it to a hydrothermal treatment performed within a specific reaction temperature range, allowed the formation of a colloidal suspension of monoclinic zirconia. at high dry content, having the properties required for an application in optical coatings, especially a high crystallinity, small particle size and a good dispersion.